1. Field of the Invention
The present invention generally relates to a method of and an apparatus for providing a multi-wavelength light source, and particularly relates to a method of and an apparatus for providing a multi-wavelength light source in which a light source comprising a number of wavelengths arranged at equal frequency spacings is produced.
2. Description of the Related Art
Ever since a Wavelength Division Multiplexing (WDM) system has been put to practical use, a trend towards a larger transmission capacity has been continuing.
Presentations on a system using hundreds of monochromatic light sources which would exceed 1 Terabit/sec in total capacity at a transmission bit rate of between 10 and 40 Gigabits/sec per wavelength are being actively carried out.
According to the specifications by the ITU-T recommendations, wavelengths of an optical signal in an optical communications system are to be arranged at 50 or 100 GHz spacings based on 193.1 THz. Therefore, the wavelengths must be controlled precisely so as to oscillate within the grid as specified therein.
As illustrated in FIG. 1, a multi-wavelength light source which has been used most generally up to now, includes monochromatic lasers 10-1 through 10-n. Herein, n is an integer. The respective monochromatic lasers may, for example, be distributed feedback lasers, etc. Such composition necessitates providing hundreds of lasers in a large-capacity communications system using hundreds of wavelengths so as to make an increase in the size of the multi-wavelength light source apparatus and an increase in the amount of electric power consumption inevitable. Also, in order to manage the absolute wavelengths, one of the monitoring and control circuits 12-1 through 12-n which monitor the output wavelengths so as to compensate for an offset from the grid must be arranged per laser.
As illustrated in FIG. 2, as another method of implementing a multi-wavelength light source in a simple composition, there is a method of implementing a multi-wavelength light source by demultiplexing at the wavelength demultiplexer 16 a longitudinal component produced by the modulation from a modulated light output at the optical pulse source 14 which is modulated at a repetition frequency of f0 (refer to the Non-Patent Document 1, for example).
The electric field E0 of a lightwave pulse train with a repetition frequency of f0 (Hz) is represented by the equation (1). Herein, t is the time, g( ) is a function, and n is an integer.
                                          E            0                    ⁡                      (            t            )                          =                              ∑            n                    ⁢                                          ⁢                      g            ⁡                          (                              t                +                                  n                  *                                      1                    /                                          f                      0                                                                                  )                                                          (        1        )            
The optical spectrum P0 may be obtained by a Fourier transform and is shown by the equation (2).
                                                        P              0                        ⁡                          (              f              )                                =                                    (                              2                ⁢                                  π                  /                                      T                    0                    2                                                              )                        ⁢                                                                                                g                    ^                                    ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                      f                                        )                                                                              2                        ⁢                                          ∑                n                            ⁢                                                          ⁢                              δ                ⁡                                  (                                      f                    -                                          n                      ⁢                                                                                          ⁢                                              f                        0                                                                              )                                                                    ,                            (        2        )            where ĝ( ) is a Fourier transform of g( ).
As it may be understood from the mathematical equation (2), the wavelength domain includes longitudinal components of f0 (Hz). Extracting these longitudinal components with a narrowband filter enables the producing of a number of monochromatic light sources, the number corresponding to only the number of longitudinal components.
This method includes an aspect to enable the implementation of a plurality of monochromatic light sources from a (pumping) light source so as to allow a simple implementation of a light source composition. Also, as the spacings between the longitudinal components correspond to the modulating frequency, managing the modulating frequency with high precision enables the leaving out of the monitoring and control of individual wavelengths.
Besides, as a technology to demultiplex a number of wavelengths all together, there is, for example, one which is described in the Patent Document 1.
Non-Patent Document 1
Optical fiber communication 2001, ME-2, Anaheim, Calif., March 17-22.
Patent Document 1
JP8-195732A
Patent Document 2
JP2002-77052A
As described above, although it is possible to extract longitudinal components of an optical spectrum with a narrowband filter, as the longitudinal components are produced by electrical modulation in the related art as illustrated in FIG. 2, their spacings are limited by the electronic operating speed. Thus there exists a problem in that producing a longitudinal component which exceeds 40 GHz, a frequency generally known as the limit of electronic operations, is difficult.